Modern electronics make heavy use of surface mount devices (SMDs) due to their small size, high component density, low manufacturing cost, and enhanced performance when compared to conventional devices such as through-hole devices. FIGS. 1A through 1C illustrate a conventional SMD 10 including a substrate 12, a number of electrical contacts 14, a thermal pad 16, and a solder mask 18 on a first surface of the substrate 12, a number of components 20 on a second surface of the substrate 12 opposite the first surface, and an encapsulant layer 22 over the second surface of the substrate 12. The components 20 may form an emitter configured to generate light in response to an electrical current. For example, the components 20 may include one or more light emitting diodes (LEDs). Accordingly, the encapsulant layer 22 may be transparent in order to pass light generated by the components 20. The electrical contacts 14 are coupled to the components 20 by one or more vias (not shown) running through the substrate 12. The electrical contacts 14 may include a first electrical contact 14A, which may be an anode contact for one or more LEDs in the components 20, and a second electrical contact 14B, which may be a cathode contact for one or more LEDs in the components 20.
Generally, SMDs are permanently attached to a carrier such as a printed circuit board (PCB) via an electrically conductive material such as solder (e.g., lead/tin solder, gold/tin solder, or any other suitable compound). FIG. 2 shows a connection 24 between one of the electrical contacts 14 and a corresponding electrical contact 26 on a surface of a carrier 28 to which the conventional SMD 10 is attached. The connection 24 may primarily be formed of solder 30, and may include an intermetallic layer 32 at the interface between each one of the electrical contact 14 and the corresponding electrical contact 26 on the surface of the carrier 28. The intermetallic layer 32 may be formed at each end of the solder 30 due to mixing of the solder 30 with a surface finish covering each one of the electrical contact 14 and the corresponding electrical contact 26 on the surface of the carrier 28, and may be a weak point of the connection 24 in many cases.
While the electrical contacts 14 are connected as required to form a circuit on the carrier 28, the remaining surface area of the conventional SMD 10 such as the thermal pad 16 is generally coupled to an electrically inactive portion of the carrier 28 in order to conduct heat away from the conventional SMD 10. Because a majority of the heat produced by SMDs is generally focused at a center point of the device, thermal pads are often located at or near the center of the device while electrical contacts are located near the outside edges of the device. The various contacts of the SMD and the thermal pad 16 are physically separated such that they are electrically isolated from one another. The solder mask 18 is optional, and may provide additional electrical isolation. The thermal pad 16 may be electrically inactive when the conventional SMD 10 is not mounted, however, in certain situations the conventional SMD 10 may be mounted such that one or more of the electrical contacts 14 are coupled to the thermal pad 16 or coupled to one another through the thermal pad 16. For example, in some cases the conventional SMD 10 may include a number of electrical contacts 14 coupled to the same or different components 20 on the second surface of the substrate 12 in order to provide different functionality of the conventional SMD 10 device depending on the particular electrical contacts 14 used to connect the device to a circuit. For example, the conventional SMD 10 may include at least four electrical contacts 14 such that a number of LEDs are coupled in series between a first pair of the electrical contacts, while at least some of the LEDs are connected in parallel between a second pair of the electrical contacts. Accordingly, the conventional SMD 10 may operate at different voltages depending on the particular configuration of the electrical contacts 14 connected to a circuit.
When the conventional SMD 10 is bonded to the carrier 28, differences in the coefficient of thermal expansion (CTE) between the substrate 12 of the conventional SMD 10, the solder 30, and the carrier 28 generate mechanical stress as the temperature of the conventional SMD 10 and/or the carrier 28 changes. This mechanical stress is lowest at the center point of the conventional SMD 10 and increases in a linear fashion with the distance from the center point as illustrated in the graph shown in FIG. 3. Accordingly, the edges of the conventional SMD 10 experience the greatest amount of stress and are the most prone to separating from the carrier 28. As mechanical stress between the various parts of the conventional SMD 10 and the carrier 28 rises above a critical point, the conventional SMD 10 separates from the carrier 28 beginning at the edges of the conventional SMD 10 and propagating inward towards the center point, as shown in FIG. 4. Generally, this separation will occur at the intermetallic layer 32 between the electrical contact 26 on the carrier 28 and the solder 30.
FIG. 4 specifically shows the conventional SMD 10 at three different points in time (T1, T2, and T3), at which larger portions of the electrical contacts 14 and/or the thermal pad 16 of the conventional SMD 10 have separated from the carrier 28. At each of T1, T2, and T3, the portions of the electrical contacts 14 and/or the thermal pad 16 outside of the respective dashed circles has detached from a corresponding contact on the carrier 28. After a portion of the electrical contacts 14 and/or the thermal pad 16 of the conventional SMD 10 has separated from the carrier 28, the separation of that particular electrical contact 14 and/or thermal pad 16 accelerates due to the fact that the connection 24 between the conventional SMD 10 and the carrier 28 has been compromised. Accordingly, once separation of the conventional SMD 10 from the carrier 28 begins, there may be little time before the conventional SMD 10 fails altogether. While the conventional SMD 10 may remain functional at T1 and T2 due to the fact that the electrical contacts 14 are still connected to the carrier 28, the conventional SMD 10 is no longer electrically connected to the carrier 28 at time T3 and thus is no longer functional. At some point in time after T3, the conventional SMD 10 may completely separate from the carrier 28.
A failure of the connection 24 between the conventional SMD 10 and the carrier 28 is dependent on many factors. Generally, a failure of the connection 24 will be accelerated due to high temperature operation and/or a large number of thermal cycles (i.e., heat up and cool down) of the conventional SMD 10 through a mechanism known as fatigue creep. Metallurgical factors such as the formation of intermetallic compounds may also weaken the connection 24 and contribute to a failure. Further, imperfections in the connection such as solder voids may alter the rate of fatigue creep, thereby affecting when a failure of the connection 24 occurs.
In light of the above, an SMD with stress mitigation measures is needed in order to reduce the failure rate of SMDs due to separation of one or more electrical contacts thereof from a PCB.